Leak detection using helium or hydrogen as tracer gas is well-known. Helium passes through the smallest of leaks in a sealed test piece. After passing through a leak in the test piece, the helium is drawn into a leak detection instrument and is measured. The quantity of helium measured corresponds to the leak rate. In one approach, the interior of a test piece is coupled to a test port of the leak detector. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector. In another approach, the test piece is pressurized with helium. A sniffer probe connected to the test port of the leak detector is moved around the exterior of the test piece. Helium passes through a leak in the test piece, is drawn into the probe and is measured by the leak detector.
To meet the demanding requirements of industry, a tracer gas leak detector needs to be capable of precise calibration and to have a low cost of ownership. The precise calibration of a leak detector may be achieved by adjusting a system gain value until the leak detector indicates the predetermined value of a calibrated leak standard, also known as a calibrated leak, having a known leak rate. In various applications, leak rates may be measured over several decades, such as 10−3 std-cc/sec to 10−9 std-cc/sec, also referred to as E-03 std-cc/sec to E-09 std-cc/sec or simply E-03 to E-09.
A prior art tracer gas leak detector is illustrated schematically in FIG. 1. A test port 30 is coupled through contraflow valves 32 and 34 to a forepump 36. The leak detector also includes a turbopump (turbomolecular vacuum pump) 40 having a fixed rotation speed. The test port 30 is coupled through midstage valves 42 and 44 to a midstage port 46 located on turbopump 40 between a foreline 48 and an inlet 50. A foreline valve 52 couples forepump 36 to the foreline 48 of turbopump 40. The inlet 50 of turbopump 40 is coupled to the inlet of a mass spectrometer 60. The leak detector further includes a test port thermocouple 62 and a vent valve 64, both coupled to test port 30, a calibrated leak 66 coupled through a calibrated leak valve 68 to midstage port 46 of turbopump 40 and a ballast valve 70 coupled to forepump 36.
In operation, forepump 36 initially evacuates test port 30 and the test piece (or sniffer probe) by closing foreline valve 52 and vent valve 64 and opening contraflow valves 32 and 34. When the pressure at the test port 30 reaches a level compatible with the foreline pressure of turbopump 40, foreline valve 52 is opened, connecting test port 30 to the foreline 48 of turbopump 40. The helium tracer gas is drawn through test port 30 and diffuses in reverse direction through turbopump 40 to mass spectrometer 60. Since turbopump 40 has a much lower reverse diffusion rate for heavier gases in the sample, it blocks these gases from mass spectrometer 60, thereby efficiently separating the tracer gas, which diffuses through turbopump 40 to mass spectrometer 60 and is measured.
In the prior art leak detector of FIG. 1, calibration is performed by using a different calibrated leak 66 for each decade of the measurement range. That is, a calibrated leak having an appropriate leak rate is selected according to the decade of the measurement range being calibrated. The selected calibrated leak is attached to the system and the signal from spectrometer 60 is measured. A difference between the measured value and the known value of the calibrated leak provides a calibration value for the selected decade of the measurement range.
A different calibrated leak is used to calibrate each decade of the measurement range in the leak detector of FIG. 1. That is, if the user will be leak testing a part in the E-05 leak rate range, the test system is calibrated using an E-05 range calibrated leak. If the user will be leak testing in the E-06 leak rate range, an E-06 range calibrated leak is used. In a production factory with many different parts being leak tested, different leak test systems may be simultaneously working in different leak rate ranges.
As stated above, the calibration process for a leak detector requires setting of a system gain value to compensate the measured leak rate to equal a known leak standard. The leak measurement capability can be scaled linearly over some range above or below the specific calibration point. However, in cases where accuracy is important, a calibrated leak standard for the specific decade of measurement is used. Calibrated leak standards cost several hundred dollars each and require periodic calibration. The cost to procure and recalibrate the leak standards and to maintain calibration tracking records is a significant cost to an industrial user.
Accordingly, there is a need for improved calibration methods and systems for tracer gas leak detection.